This invention relates to a polyimide copolymer film for use as a lift-off layer in high-temperature metallization of integrated circuit substrates.
In order to achieve higher LSI packing densities and speeds, highly packed multilevel interconnection metallization is very desirable. Generally, the definition of the metallization pattern is accomplished by conventional wet chemical etching techniques. However, fabrication technologies, such as dry etching or lift-off metallization have been developed because the conventional chemical etching technology limits the feature size of the metallization pattern.
Although dry etching is very effective in obtaining fine patterns, dry-etched metallization patterns make it very difficult to produce high performance, multilevel interconnection metallization. The steep, sharp edges of the metallization profile obtained by dry etching results in poor step coverage for subsequently deposited layers.
Polymer lift-off films continue to find increased use in the patterning of relief metal interconnections in wafer (semiconductor) and packaging (high density interconnect) systems. In lift-off metallization, a patterned relief layer of photoresist is first formed on a substrate. A layer of metal for integrated circuit conductor lines is coated over the resist layer and the exposed portions of the substrate. The resist layer is then stripped off and takes with it the overlying metal to leave only the pattern of metal in direct contact with the substrate. The lift-off process takes advantage of the natural undercutting of the resist during high energy exposure such that the developed resist pattern is wider at the bottom than at the top. This profile aids in forming a discontinuity between the portions of metal which are on the substrate surface and the portions which cover the resist. This discontinuity is needed in order to permit the resist stripping solution to attack the unexposed resist and remove it along with the overlying metal.
Lift-off metallization produces fine-featured tapered metallization patterns which are favorable for achieving good step coverage. The most serious drawback of this technology is that the metallization layer must be deposited at low temperatures because of the poor heat resistance of the conventional photoresists, such as polymethyl methacrylate, used as the lift-off layer. This worsens the electrical characteristics of the circuit devices and the yield of the metallization patterns.
U.S. Pat. No. 3,873,361, issued to Franco et al on Mar. 25, 1975, discloses a lift-off method for depositing thin metal films in the fabrication of integrated circuits comprising depositing an organic polymeric material on the integrated circuit substrate and an overlying metal masking layer having openings in the selected pattern. Openings are formed in the polymeric material by reactive sputter etching utilizing the metallic mask as a barrier. The openings in the polymeric layer are aligned with and are laterally wider than the corresponding openings in the metallic masking layer as a consequence of the reactive sputter etching step. Thus, the edge of the openings in the metallic masking layer overhang the edges of the openings in the underlying polymeric layer. The thin metal film is then deposited over the structure and on the surface of the substrate exposed by the openings in the polymeric material. When the polymeric material is removed by application of solvent, the metallic masking layer and the thin metal film above the masking layer "lift-off" to leave the thin metal film deposits in the selected pattern on the substrate without tearing the edges of the desired deposited thin metal film as the unwanted portions of the metal film are lifted off.
U.S. Pat. No. 4,202,914, issued to Havas et al on May 13, 1980, further discloses an improved three-layer lift-off process for depositing thin metal films on an integrated circuit substrate or wafer comprising first covering the substrate or wafer with a first masking layer of an organic polymeric photosensitive material and then depositing a layer of silicon nitride on the first masking layer. The silicon nitride layer is covered by a second masking layer of organic polymeric resist material through which apertures are formed in preselected patterns using standard lithographic masking and etching techniques. The silicon nitride layer is then reactive ion etched with CF.sub.4 through the apertures formed in the second masking layer. The second masking layer is removed and the first organic polymeric masking layer is then reactive ion etched with oxygen through the apertures in the silicon nitride layer. The etching of the first organic polymeric masking layer continues until the first masking layer is undercut beyond the edges of the aperture in the silicon nitride layer so that the silicon nitride forms an overhang of the aperture in the first masking layer. The layer of metal film is then evaporated over the entire exposed surface of the structure, including the surface of the silicon nitride layer and the substrate exposed through the apertures. Because of the overhang, a discontinuity is formed between the thin metal film deposited upon the exposed surface of the substrate and that formed upon the outer surface of the silicon nitride layer so that when the first masking layer is dissolved, the metal film deposited upon the substrate is left without any edge tearing between it and the removed portions of the metal film.
The organic polymeric material used as the liftoff layer in the prior art patents must exhibit good adhesion to both the substrate and the silicon nitride layer, be thermally stable and be removable by reactive sputter etching. Preferred materials include Novalac-type phenol-formaldehyde resin and a photosensitive cross-linking agent, polyvinyl cinnamate, polymethyl methacrylate and polyimides such as the reaction product of pyromellitic dianhydride and 4,4'-oxydianiline.